In the prior art there are numerous lunar phase display devices, especially incorporated in watches.
By way of example, EP Patent No 2392976 discloses a device comprising a moon disc and a dial comprising an aperture of substantially semi-circular shape. The moon disc is driven in rotation to display the indications marked on the moon disc through the aperture to indicate the phases of the moon. This mechanism includes a first occultation disc rotatably housed between the moon disc and the dial, and a drive train driving the first occultation disc such that the indications marked on the moon disc are at least partially obscured during certain operating phases of the mechanism so that the indications that appear through the aperture correspond to the natural appearance of the moon throughout the duration of the lunation.
CH Patent No 706094 discloses a moon phase display device comprising a dial with an axis, a moon indicator, a cover positioned to conceal the moon indicator in varying degrees, and first rotational drive means for the moon indicator to move the moon indicator in rotation.
These devices can satisfactorily reproduce the appearance of the moon. However, they are complicated and of great thickness.